Osteoporosis is defined as a rapid reduction in bone mass. With the growing longevity of the population, there is a substantial increase in the frequency of this disorder. Bones are in a constant state of remodeling which consists of two processes: bone resorption and bone formation, performed by the osteoclasts and osteoblasts, respectively. With age, the thickness of the born gradually declines, primarly because the osteoblasts are unable to keep pace with the osteoclasts. Determining the key regulatory mechanisms involved in maintaining the integrity of the osteoblast is important for understanding the aging process. This is a proposal to investigate the control of osteoblast-specific gene expression. A novel molecular/genetic experimental system will be utilized to investigate the regulation of four osteoblastic genes: estrogen receptor, bone Gla protein, matrix Gla protein and liver/bone/kidney alkaline phosphatase. A set of intertypic hybrids will be constructed between highly differentiated rat osteosarcomas and human fibroblasts. Genetic studies of this nature have been utilized to identify tissue-specific trans-dominant negative regulatory elements termed extinguishers in a number of experimental system. The goal of this project will be to identify and localize extinguishers which control the expression of osteoblast-specific genes. In addition, experiments are designed to identify the Cis sequences in the osteooblast-specific structural gene that are required for their regulation. By identifying and ultimately isolating these unique regulators, we can begin to understand their role in the development and maintenance of the osteoblast. Important insights can be gained into perturbations of these regulatory circuits which might occur in the osteoblast during aging.